Methanol oxidation over bulk metal vanadate catalysts

ABSTRACT

A method wherein a methanol-containing gas stream is passed in contact with a catalyst comprising a supported or unsupported bulk vanadate catalyst in the presence of an oxidizing agent for a time sufficient to convert at least a portion of the methanol to formaldehyde (CH 2 O).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119 and 119(a) ofprior filed U.S. provisional application No. 60/341,284, filed Dec. 20,2001, the entirety of which is hereby incorporated by reference, andpriority of PCT application no. US02/40747, filed Dec. 20, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention broadly relates to a catalytic composition suitable forthe selective oxidation of methanol to formaldehyde. The invention isspecifically directed to compositions comprising bulk metal vanadatesand particularly to the use of such compositions as catalysts forselectively oxidizing methanol to formaldehyde.

2. Description of Related Art

The formation of formaldehyde involves the selective oxidation ofmethanol. One approach for converting methanol to formaldehyde involvesoxidizing methanol over a silver catalyst. See, for example, U.S. Pat.Nos. 4,080,383; 3,994,977; 3,987,107; 4,584,412; 4,343,954 and4,343,954. Typically, methanol oxidation to formaldehyde over a silvercatalyst is carried out in an oxygen lean environment. One problemassociated with silver catalyzed methanol oxidation is methanol leakage,i.e., high amounts of unconverted methanol.

An alternative process, which uses a methanol/air mixture (e.g., areactant gas feed stream of methanol, excess air and an inert carriergas) introduced over an iron-molybdate/molybdenum trioxide-typecatalyst, also finds widespread use. See, for example, U.S. Pat. No.3,983,073 (conversion of methanol to formaldehyde using Fe₂(MoO₄)₃ andMoO₃ having a molar ratio of Mo/Fe from 1.5 to 1.7 and a degree ofcrystallinity of at least 90%); U.S. Pat. No 3,978,136 (process for theconversion of methanol to formaldehyde with a MoO₃/Fe₂O₃/TiO₂ catalystwherein the MoO₃:Fe₂O₃ weight ratio is between 1:1 to 10:1 and TiO₂ ispresent between 1 to 90 weight % of total oxides); U.S. Pat. No.3,975,302 (a supported iron oxide and molybdenum trioxide catalystwherein the atomic ratio of Mo/Fe is from 1.5 to 5); U.S. Pat. No.3,846,341 (a shaped and optionally supported iron molybdate typecatalyst having high mechanical strength made by reacting ammoniummolybdate and ferric molybdate); U.S. Pat. No. 3,716,497 (an optionallyshaped iron molybdate type catalyst made by admixing with NH₄ ⁺A⁻); U.S.Pat. No. 4,829,042 (high mechanical strength catalyst of Fe₂(MoO₄)₃ andMoO₃ together with non-sintered Fe₂O₃); U.S. Pat. No. 4,024,074(interaction product of Fe₂(MoO₄)₃, MoO₃ and bismuth oxide forcatalyzing oxidation of methanol to formaldehyde); U.S. Pat. No.4,181,629 (supported catalyst of iron oxide and molybdenum oxide onsilica, alumina and the like); U.S. Pat. No. 4,421,938 (a supportedcatalyst of at least two oxides of Mo, Ni, Fe and the like); and U.S.Pat. No. 5,217,936 (a catalyst of a monolithic, inert carrier and oxidesof molybdenum, iron and the like).

In comparison to the silver catalyzed processes, theiron-molybdate/molybdenum trioxide catalyzed processes produce higheryields of formaldehyde and would appear to be preferred approach.Iron-molybdate, Fe₂(MoO₄)₃, in combination with molybdenum trioxide,MoO₃, constitute the metal oxide phases of exemplary commerciallyavailable metal oxide catalysts suitable for oxidizing methanol toformaldehyde. Typically, such catalysts used in industrial andcommercial applications contain an excess of MoO₃. Thus, for example,the molar ratio MoO₃/Fe₂O₃ may vary from 1.5/1 to 12/1 or more. ExcessMoO₃ is provided to cover the surface iron sites with a monolayer ofmolybdenum species for efficiently oxidizing methanol to formaldehyde inhigh yields.

Thus, much of the formaldehyde produced by oxidizing methanol isprepared by reacting methanol with oxygen over this bulk MoO₃/Fe₂(MoO₄)₃mixture.

Processes that use a bulk MoO₃/Fe₂(MoO₄)₃ catalyst are generallyconducted using a tubular-type reactor having 10,000 to 20,000 tubes andthe reaction is conducted at a temperature of 300–360° C. and typicallyobtains about an 88% yield of formaldehyde.

Processes that use a bulk MoO₃/Fe₂(MoO₄)₃ catalyst are not free ofproblems, however. Oxidizing methanol to formaldehyde using a metalmolybdate/molybdenum trioxide type catalyst, e.g., Fe₂(MoO₄)₃/MoO₃, is ahighly exothermic process. Heat released during the oxidation reactionincreases the catalyst and/or the fixed bed reactor temperatureproducing hot spots on the catalyst surface. These hot spots reachtemperatures high enough to volatilize Mo/MoO₃ species present withinmetal molybdate/molybdenum trioxide type catalysts. Additionally, in thepresence of the methanol reactant, a volatile compound is generatedbetween molybdenum and methanol (Mo—OCH₃). Thus, Mo/MoO₃ is sublimed, ora volatile molybdate compound is generated, as a consequence of such hotspots, and contributes to several adverse consequences.

The Mo/MoO₃ species migrate downstream (e.g., within an exemplary fixedbed reactor housing the catalyst) towards cooler regions of the fixedbed reactor or the like. Typically, the downstream migration of Mo/MoO₃species is facilitated by the incoming flow of the reactant gas feedstream. The migrated Mo/MoO₃ species crystallize in the coolerdownstream regions of the fixed bed reactor, for example, in the form ofMoO₃ crystalline needles. Over time, the needle formation accumulatesand ultimately obstructs the flow of the reactant gas feed streamthrough the fixed bed reactor. Thus, build up of MoO₃ crystals/needlesin the downstream region causes a substantial pressure drop in thereactant gas feed stream flow rate as the reactant gas feed stream isdirected downstream. This pressure drop impedes the efficient oxidationof methanol to formaldehyde. See, for example, U.S. Pat. No. 3,983,073(col. 1, lines 35–52); and U.S. Pat. No. 4,024,074 (col. 1, lines60–68); and U.K. Patent No. 1,463,174 (page 1, col. 2, lines 49–59)describing the aforementioned volatility problem.

Often, the MoO₃ needle formation that occurs in the downstream region ofthe fixed bed reactor is so excessive that the reactor must be shutdown, the needles cleaned out, and fresh catalyst charged therein. Thesesteps unnecessarily increase the time, cost, inefficiency and/orcomplexity of operating a fixed bed reactor or the like for oxidizingmethanol to formaldehyde.

Even before such severe problems occur, the loss of the Mo/MoO₃ speciesfrom one location and their migration to another location createsregions where surface iron sites are exposed thus diminishing thecatalytic activity and selectivity of the catalyst towards formaldehyde.This not only reduces the yield of the desired formaldehyde, but alsoincreases the production of environmental hazardous gases such usCO_(x).

While pending U.S. application Ser. No. 09/950,832 provides one approachfor ameliorating this problem, there remains a need to providealternative catalysts suitable for selectively oxidizing methanol toformaldehyde.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a process for selectively oxidizingmethanol to formaldehyde using compositions comprising bulk metalvanadates. According to the invention, a reactant gas stream containingmethanol is passed in contact with a catalyst comprising a supported orunsupported bulk metal vanadate catalyst in the presence of an oxidizingagent; typically oxygen. The gas stream is contacted with the catalyst,in the presence of the oxidizing agent, for a time sufficient to convertat least a portion of the methanol to formaldehyde.

The bulk metal vanadate catalyst compositions useful for practicing thepresent invention are known in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a methanol-containing gasstream is contacted, under an oxidizing condition, with a bulk metalvanadate catalyst composition. The catalyst can either be unsupported,or supported on a substrate. The oxidizing conditions are selected tofacilitate partial oxidation of the methanol in the stream toformaldehyde.

The gas stream containing methanol contacts the bulk metal vanadatecatalyst under oxidizing conditions at a temperature in the range of200° to 700° C., preferably in the range of 250° to 600° C. and mostoften in the range of 275° to 450° C. The oxidizing agent can usually beoxygen or air. The contacting of the methanol-containing gas with thebulk metal vanadate catalyst under an oxidizing atmosphere, e.g., in thepresence of oxygen, and at an appropriate temperature, causes aselective conversion of the methanol to formaldehyde. The gaseous feedstream generally will comprise at least about 0.1 mole %, and preferablyat least 1.0 mole % and higher of methanol, although higherconcentrations may be employed. The gas stream may also include water.The gas stream preferably contacts the catalyst at a temperature ofabout 275° to 450° C.

To achieve high selectivity in the conversion of methanol toformaldehyde, it is important to maintain the flow rate of the gasstream to provide an amount of methanol per unit mass of catalyst in therange of 10⁻² to 10⁵ cubic centimeters of methanol (assessed understandard conditions of temperature and pressure (STP)) per gram ofactive catalyst per minute (excluding inert ceramic components or otherinert catalyst support material). Generally, higher reactiontemperatures permit higher flow rates. Usually, the process can beoperated at 0.1 to 10⁴, cubic centimeters (STP) of methanol per gram ofcatalyst per minute.

As used herein, the term “selectively” is intended to embrace theconversion of at least 1% of the methanol, preferably at least 10% ofthe methanol, more usually at least 50% of the methanol and most oftenat least 70%, and most preferably 95% of the methanol, which contactsthe catalyst, to formaldehyde. Selectivity (expressed as a percentage),as that term is used herein, is determined by dividing the moles offormaldehyde in the methanol conversion products by the moles ofmethanol converted (consumed) from the feed to the reactor times 100.Selectivity indicates the percentage of formaldehyde formed as comparedto the percentage of non-formaldehyde oxidation products of methanolsuch as CO, CO₂, dimethoxymethane (DMM), dimethyl ether (DME), etc. Asused herein, the term conversion is determined by dividing thedifference between the number of moles of methanol fed to the fixed bedreactor in the reactant gas feed stream minus the number of moles ofmethanol exiting the reactor by the total number of moles of methanolfed times 100. As with selectivity, conversion also is a percentagevalue. Conversion indicates the percentage of the moles of methanol thatwere oxidized to formaldehyde and any other non-formaldehyde oxidationproducts of methanol. Thus, if 2 moles of methanol are fed into thefixed bed reactor (e.g., in a reactant gas feed stream) yielding 1 moleof formaldehyde and 1 mole of methanol, then selectivity would equal100% while conversion would equal 50%. Likewise, if 3 moles of methanolare fed into the fixed bed reactor (e.g., in a reactant gas feed stream)yielding 2 moles of formaldehyde and 1 mole of methanol, thenselectivity would equal 100% while conversion would equal 66 and ⅔%.

The oxidation reaction is exothermic. As recognized by those skilled inthe art, a variety of reactor designs, such as tubular reactors, may beemployed to accommodate the necessary mass and heat transfer processesfor effective operation of the process on a continuous basis. Thereaction may be conducted at atmospheric pressure and above, or belowatmospheric pressure. Suitable exemplary reactor temperatures range fromabout 300° C. to about 450° C. Suitable exemplary reactor pressuresrange from about 7 psia (i.e., about ½ atm) to about 165 psia. Suitableexemplary reactant gas space velocity ranges from about 0.5 sec⁻¹ toabout 3.0 sec⁻¹. Other conditions suitable for oxidizing methanol toformaldehyde are used which are well known to those of ordinary skill inthe art.

The partially oxidized reactant feed gas stream is hereafter referred toas the product gas stream. Formaldehyde (FA) is a significant componentof the product gas stream together with quantities of one or more ofsome unreacted methanol (if any), water vapor or condensed water inaerosol form or the like, an inert carrier gas (if any), oxygen, andother products such as DMM (dimethoxy methane), DME (dimethyl ether),methyl formate (MF), CO, CO₂ and the like.

Formaldehyde is the intended product and it can be recovered from thegaseous reaction products using any one of a number of ways known tothose skilled in the art.

As will be recognized by those skilled in the art, the gases leaving thereactor may contain unreacted methanol, and will contain inert gasesthat may have been added, as well as formaldehyde and water. Theprincipal by-product that is formed during the partial oxidation ofmethanol is carbon monoxide, which may be accompanied by a small amountof carbon dioxide.

The reaction mixture leaving the catalytic reactor is generally subjectto further processing in a conventional manner. For example, theformaldehyde product can be separated in a washer (absorber), or byindirect cooling, or also by fractional cooling. For example, thewashing can be performed with water, in which case a multi-stage washercan be used. An aqueous formaldehyde solution is obtained in thismanner. From this solution commercial formaldehyde solutions can beprepared by distillation for immediate technical use. The formaldehydealso can be condensed out of the reaction gas together with the waterthat has formed. In this manner, concentrated formaldehyde solutions incommon commercial form eventually can be obtained. Other ways forisolating and recovering the formaldehyde product will be apparent tothose skilled in this art and the present invention is not limited toany particular isolation and recovery technology.

Suitable bulk metal vanadate catalysts for use in connection with thepresent invention are known. Suitable metal vanadate catalysts will, inaddition to vanadium, contain a wide variety of other metal species suchas alkali metals (such as sodium (Na), lithium (Li), potassium (K) andcesium (Cs)), alkaline earth metals (such as calcium (Ca), barium (Ba),and magnesium (Mg)) and transition metals (such as copper (Cu), nickel(Ni), cobalt (Co), aluminum (Al), lead (Pb), bismuth (Bi), iron (Fe),zinc (Zn), cadmium (Cd), tellurium (Te), manganese (Mn)). Suitable bulkmetal vanadate catalysts for use in connection with the presentinvention thus include, as non-limiting examples: PbV₂O₆, Pb₂V₂O₇,NaVO₃, Na₃VO₄, Na₂V₆O₁₇, BiVO₄, Bi₄V₂O₁₁ and other Bi—V—O familymembers, AlVO₄, FeVO₄, Mn₃(VO₄)₂, Mg₂(VO₄)₂, Mg₂V₂O₇, MgV₆O₁₇, MgV₂O₆,CeVO₄, Zn₃(VO₄)₂, Zn₂V₂O₇, CdV₂O₇, VOPO₄, (VO)₂P₂O₇ and other V—P—Ofamily members, KVO₃, K₂V₆O₁₇, (NH₄)₂V₆O₁₇, NH₄VO₃, BaV₆O₁₇, Tl₃VO₄,TlVO₃, TlV₃O₈, Tl₃V₅O₁₄, Tl₄V₂O₇, Ti_(1−x)V_(x)O₂, TiVO₄, TiV₂O₆,TiV₄O₁₀, NbVO₅, Nb₂V₂O₉, Nb₉VO₂₅, CrVO₄, Ni₃(VO₄)₂, Co₃(VO₄)_(2, Co)₃V₂O₈, AgVO₃, Cu₃(VO₄)₂, Cu₂V₂O₇, Cu₃V₂O₈, CuVO₃, CuV₂O₆, SbVO₄, VSbO₄,Sn_(1−x)V_(x)O₂, SrVO₄, Sr₂V₂O₇, CsVO₃, RbVO₃, ZrV₂O₇, V₃Zr₃O, NdVO₄,SmVO₄, EuVO₄, YVO₄, LaVO₄, ReVO₄, LiVO₃, LiV₃O₈, Ca(VO₃)₂, CaV₆O₁₇,Ca₆V₁₀O₂₈, Ca₂V₂O₇, Hg(VO₃)₂, VOSO₄ and mixtures thereof.

Additionally, bulk and supported isopoly and heteropoly-oxometalates areconsidered as suitable materials for methanol selective oxidation toformaldehyde. The structures considered are Keggin XM₁₂O₄₀ ⁴⁻,Wells-Dawson [(X^(n+))₂M₁₈O₆₂]^((16−2n)−) and Anderson XO₆M₆O₁₈ ^(n−)type anions where X^(n+) represents a central atom [phosphorous (V),arsenic (V), sulfur (VI), fluorine, aluminum (III), silicon (IV), iron(II), cobalt (II), copper (II), zinc (II), manganese (II), tellurium(VI), gallium (III), nickel (II), chromium (III), cobalt (II) andothers] surrounded by a cage of M addenda atoms, such as tungsten (VI),molybdenum (VI), vanadium (V) or a mixture of elements, each of themcomposing MO_(x) (M-oxygen) units. The addenda atoms are partiallysubstituted by other elements, such as vanadium, transition metals,lanthanides, halogens and others. The heteropoly-anions are associatedwith inorganic (protons, alkaline elements and others) or organiccountercations generating heteropoly acids and salts.

Methods for making bulk vanadates used in the present invention also areknown to those skilled in the art. In particular, the active catalystcan be prepared by physically blending and grinding metal oxides, bycoprecipitation from aqueous and non-aqueous solutions containingsoluble compounds of the catalyst components in the desired molar ratio,by thermal transformation, by sol-gel formation or by any othertechnique that provides an intimate mixture of the vanadateconstituents. For example, an aqueous solution of a water-solublevanadium compound (e.g., ammonium metavanadate) is mixed with awater-soluble metal compound (e.g., ferric nitrate) and the solution ismodified (e.g., by pH adjustment such as acidification) to causecoprecipitation of both vanadium and iron, using procedures well knownto those skilled in the art. The coprecipitate can be washed toeliminate soluble salts formed during the coprecipitation reactions,filtered, dried, and then is calcined to convert the metal constituentsto their active metal vanadate (oxide) form. Those skilled in the artrecognize a variety of water soluble metal compounds that can be used toprepare the active catalyst. Alternatively, oxides of the respectivemetals may be ground together and calcined. Additional details on bulkvanadates and bulk vanadate catalysis can be found in Arora et al.,Journal of Catalysis, 159, (1996) 1–13 (bismuth vanadates) and Wachs, I.E., Ed., Characterization of Catalytic Materials, Chapter 3 “Bulk MetalOxides,” pp. 47–68 (1992 ), which are incorporated herein by reference.

Bulk vanadate catalysts are crystalline in nature, possess long rangeorder, and give rise to an x-ray diffraction (XRD) pattern. Thecrystalline form can also usually be detected with Raman spectroscopy(often more sensitive than XRD). Further, information concerning bulkmetal oxide catalysts in general may be found in J. Raman Spectroscopy,21, 683–691 (1990); J. Physical Chemistry, 95(13), 5031–5041 (1991);Solid State Ionics, 45, 201–213 (1991); J. Raman Spectroscopy, 26,397–405 (1995); and J. Chem., Soc., Faraday Trans., 92(11), 1969–1973(1996), and Characterization of Catalytic Materials, edited by Israel E.Wachs, Chapter 3, pp. 47–68 (Butterworth-Heinemann, 1992) all of whichare incorporated herein by reference.

In preparing a suitable bulk vanadate catalyst, a period of thermaltreatment is generally necessary to convert catalyst precursor speciesto active bulk catalyst. Such treatment can occur either duringcalcination or under reaction conditions, or using some combinationthereof. Under these conditions the catalyst precursor components aretransformed into the active bulk catalyst. Suitable catalyst speciesappear to be formed as a result of calcination at about 350° to about850° C., preferably about 35020 to about 700° C. and most preferablyabout 400° to about 600° C., for a period of at least about 0.5 hour,preferably for a period of about 2 to about 3 hours. The time period maydepend on equipment used, as known to those skilled in the art.

As noted above, in the broad practice of the present invention, the bulkvanadate catalyst can be either unsupported or supported. Methods fordispersing the active catalyst on a suitable support material are known.The support material usually comprises a porous refractory oxide orheteropolyoxometalate. Preferred are refractory oxides and other similarmaterials having a specific surface area of at least about 1 m²/g. Mostsupports will have a specific surface area in the range of 1–20 m²/g.Suitable support materials include such refractory oxides as zirconia,silica-alumina, magnesium oxide, alumina-silica-magnesia,silica-zirconia, alumina, silica, titania (titanium dioxide),silica-titania, silica-magnesia, silica-zirconia-titania and othercombinations of such materials. Also available as supports are amorphousand crystalline alumino-silicates, both natural and synthetic, andcrystalline silicas. Most often, the support used in the invention willbe relatively inert (does not adversely affect the catalyzed reactions)with respect to the catalytic composition dispersed thereon. Oxidessupported on high surface area materials such as silica, alumina orrefractory monoliths are commercially available. Silica often may be thebest support for the bulk vanadate.

The unsupported or supported catalyst, in turn, can advantageously beprovided as a coating on a foamed ceramic, honeycomb or a monolithiccarrier, such as those having a unitary cylindrical body with aplurality of fine, substantially parallel gas flow passages extendingtherethrough and connecting both end-faces of the carrier to provide a“flow-through” type of carrier. Such carriers may be prepared with knownceramic-like materials such as cordierite, silicon nitride, mullite,spodumene, sillimanite, petalite, and silica-carbide. Typical monolithiccarriers are thin-walled channels which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval and circular. Such structures may containfrom about 60 to 600 or more gas inlet openings (“cells”) per squareinch of cross section. The active supported or unsupported catalyst mayalso be provided as a layer on refractory particles such as spheres,ceramic rings, pellets or short, extruded segments of a refractorymaterial such as alumina.

A supported bulk vanadate can be prepared in a variety of ways asrecognized by those skilled in the catalyst art. For example, an aqueousslurry of a particulate bulk vanadate (or a precursor thereof) can beapplied to the support, dried and heated (calcined) to form (adhere) acatalytic material coating. The coating slurry can be prepared by mixingthe vanadate particles or precursor particles with water andball-milling (pulverizing) the mixture to a desired particle size. Thecoating of catalytic material may be applied by dipping the support intothe aqueous slurry of the catalyst or catalyst precursor particles.Alternatively, the catalyst precursor species, as a solution, can beincorporated onto the support by known impregnation and co-precipitationtechniques, wherein the desired catalyst species are formed in part byco-precipitation directly onto the suitable support.

Preparation of active bulk vanadate catalyst in the form of pills,pellets, granules, rings, spheres and the like by comulling techniquesalso is known. Particulate bulk metal oxide or metal oxide precursorspecies optionally may be combined with an inorganic clay binder,optionally a support material and the necessary amount of water to forma paste or dough which is extruded or pelletized, dried and heat treated(calcined) to yield active catalyst of a desired extrudate form andstrength. As understood by those skilled in the art, the physicalproperties of the extruded materials (density, macroporosity and surfacearea) depend on a variety of parameters.

It often is desired that the bulk vanadate used in accordance with thepresent invention have a surface area in the range of about 0.1 to about150 m²/g and higher. Use of free bulk vanadate particulates might bedesirable when large catalyst volumes are needed or if the catalyst bedis operated in a fluidized state. A monolithic form or deposition of theactive bulk catalyst on a catalyst support, such as on an inert ceramicsupport, might be preferred in applications where catalyst movement isto be avoided because of concerns about catalyst attrition and dusting,and a possible increase in pressure drop across a particulate bed. In apreferred approach, a bulk vanadate supported catalyst, may use aceramic or refractory inorganic carrier such as silicon carbide, siliconnitride, carborundum, steatite, alumina and the like, provided in theshape of rings or pellets. Typically, the active catalyst will beapplied to a support, including an inert ceramic support in an amount toprovide 1 to 20% by weight, and preferably 5 to 15%, of the supportedcatalyst.

As noted, the oxidizing agent used in the selective oxidation canusually be oxygen or air. The contacting of methanol with the bulkvanadate catalyst under an oxidizing atmosphere, e.g., in the presenceof oxygen, and at an appropriate temperature, causes a selectiveoxidation of the methanol to formaldehyde.

EXAMPLES

To facilitate a more complete understanding of the invention, a numberof examples showing catalyst preparation and use are provided below. Thescope of the invention, however, is not limited to specific embodimentsdisclosed in these examples, which are for purposes of illustrationonly. In the examples the various metal salts were obtained either fromAlfa Aesar or J. T. Baker at a purity of 99.9%.

Catalyst Preparation and Characterization—Bulk vanadate catalysts can beprepared as follows:

Preparation Example 1

Magnesium Vanadate (Mg₂(VO₄)₂)

A bulk magnesium vanadate catalyst composition was prepared inaccordance with the following procedure. 5 g of hydrated magnesiumnitrate (Mg₃(NO₃)₂.6H₂O) were dissolved in 200 ml of distilled water towhich citric acid was added and the mixture was stirred to dissolve thenitrate. Citric acid was added in a quantity sufficient to ensure thatthe molar number of equivalent anions equaled that of cations(typically, 5–8 g.). An amount of ammonium metavanadate (Alpha AesarProducts 99.9%) to satisfy stoichiometry with respect to the magnesiumsalt was separately dissolved in 200 ml of distilled water and thesolution was added to the magnesium nitrate-citric acid solution. Themixture was dried in a steambath until a glassy textured solid(precursor) was observed. The precursor was further dried in a vacuumoven overnight at a temperature of 70° C., ground and calcined in orderto obtain crystalline material. The calcination was conducted at 600° C.for 4 hours.

The purity of the vanadate phase was determined by Raman spectroscopy.The spectra were obtained under ambient conditions with an Ar⁺ ion laser(Spectra Physics Model 2020-50, excitation line 514.5 nm) delivering15–40 mW of incident radiation. Powdered vanadate (aprox. 100–200 mg)was pressed into a thin wafer about 1 mm thick that was mounted onto aspinning sample holder and rotated at 2000 rpm to avoid local heatingeffects. Scattered radiation from the sample was directed into a SpexTriplemate spectrometer (Model 1877) coupled to a Princeton AppliedResearch OMA III optical multichanneled analyzer (Model 1463) equippedwith an intensified photodiode array detector cooled to 243° K. Thespectral resolution and reproducibility within 2 cm⁻¹.

The BET surface area of the vanadate was determined by N₂ adsorption at77° K on a Quantasorb surface area analyzer (Quantachrome Corporation,Model OS-9). The magnesium vanadate exhibited a surface area (by BET) ofabout 24 m²/g.

Preparation Examples 2–11

Selective Metal Vanadates

Using the procedure of Preparation Example 1 and the salts identified inTable 1, a variety of the corresponding bulk vanadates were prepared. Inaddition to the starting salt and the resulting vanadate composition,Table 1 also provides the calcination time and calcination temperature,as well as the result of the BET surface area determination.

TABLE 1 Calcination Calcination Temperature Time S_(BET) ExampleCatalyst Precursor Salt (° C.) (Hr.) (m²/g) 1 Mg₃(VO₄)₂ Mg(NO₃)₂.6H₂O600 4 24.2 2 AgVO₃ AgNO₃ 400 48 0.8 3 NbVO₅ Nb(HC₂O₄)₅ 500 12 15.6 4Cu₃(VO₄)₂ Cu(NO₃)₂.6H₂O 500 4 3.3 5 CrVO₄ Cr(NO₃)₃.9H₂O 550 4 14.8 6Mn₃(VO₄)₂ MnCl₂.4H₂O 550 4 3.1 7 AlVO₄ Al(NO₃)₃.9H₂O 600 48 8.4 8Ni₃(VO₄)₂ Ni(NO₃)₂.6H₂O 550 4 15.3 9 Co₃(VO₄)₂ Co(NO₃)₂.6H₂O 500 5 8.910 FeVO₄ Fe(NO₃)₃.9H₂O 550 4 4.8 11 Zn₃(VO₄)₂ Zn(NO₃)₂.6H₂O 500 4 5.2

Examples 1–11

Determining Surface Active Sites and Sites and Methanol Oxidation

The numbers of active surface sites (Ns) of the various vanadatecatalysts, prepared and characterized as described above, werequantified by methanol chemisorption. The catalyst was exposed to amixture of 2000 ppm of methanol vapor in He at 100° C. to generate astable monolayer of surface methoxy species. Methoxy species M—OCH₃ arethe intermediates species in the production of partially oxygenatedreaction products during methanol selective oxidation over the bulkcatalyst. Therefore, the knowledge of the amount of surface methoxyspecies formed during methanol chemisorption is key for determining thenumber of surface active sites available for methanol selectiveoxidation. A detailed flow diagram of the equipment and thechemisorption technique has been published. See Briand, L. E., et al.,Catal. Today 62 (2000) 219–229 and Briand, L. E., et al., J. Catal. 202(2001) 268–278. The specific reaction rate (TOF) then can be calculatedby determining the production rate of redox products and normalizing therate to the number of surface sites available for adsorption. Theturnover frequency TOF is the number of methanol moles converted per molof active surface site per second.

Methanol oxidation steady state kinetics of the various vanadatecatalysts, prepared and characterized as described above, also wereobtained in a fixed-bed catalytic reactor under differential conditions(methanol conversion ≦10%) and also at high methanol conversion. Thefollowing operating parameters were used to maintain methanol conversionbelow 10% for methanol reaction over the vanadate catalyst: sampleweight, ˜10 mg, reaction temperature, 300° C.; flow rate, 100 cm³ (NTP)min⁻¹ and feed gas composition methanol/oxygen/helium, 6/13/81 mol %.

The experiments conducted at a high methanol conversion were performedunder the following operating conditions: sample weight, 30–200 mg,reaction temperature, 300° C.; flow rate, 100 cm³ (NTP) min⁻¹ and feedgas composition methanol/oxygen/helium, 6/13/81 mol %.

The catalyst was tested for 24 hs. at the high methanol conversion inorder to determine the stability under reaction conditions. Thecatalysts were stable under these reaction conditions. Methanolconversion and the amount of products (FA: formaldehyde, DMM: dimethoxymethane, MF: methyl formate) were quantified with an on-line gaschromatograph (HP 5840) equipped with TCD and FID detectors and twocolumns: capillary column (CP-sil 5CB) for methylal, dimethyl ether,methyl formate and methanol analysis and a packed column(Carboxene-1000) for CO, CO₂, O₂, formaldehyde and methanol analysis.

The results of the determinations of the number of surface active sites(Ns), the reaction rates, TOFs and selectivities of the various bulkmetal vanadates of Preparation Examples 1–11 toward methanol selectiveoxidation at low conversions are shown below in Table 2. Table 3presents the selectivity results for selected catalysts at high methanolconversions.

A particular advantage of the process of the present invention presentsis that the bulk vanadate catalysts are more active than bulk metalmolybdates and thus can be used at a lower temperatures of reaction thanthe current industrial processes using bulk molybdate catalysts whileretaining similar yield (selectivity) to formaldehyde.

Another advantage of the bulk metal vanadates is that they do notdecompose under reaction conditions (no excess of V₂O₅ is required).Therefore, the problems related to the catalyst deactivation and reactorplugging by V₂O₅ needles are avoided.

TABLE 2 Reaction rate^(a) TOF^(b) Ns 300° C. 300° C. SELECTIVITY %EXAMPLE Catalyst [μmol/m²] [μmol/m²sec] [sec⁻¹] FA DMM DME MF CO₂ 1Mg₃(VO₄)₂ 0.56 0.80 1.43 95.5 — — 4.5 — 2 AgVO₃ 21.14 35.7 1.56 92.5 — —— 7.4 3 NbVO₅ 1.61 5.1 3.14 87.9 — 3.2 — 8.5 4 Cu₃(VO₄)₂ 0.98 6.1 6.2195.0 — — 5.0 — 5 CrVO₄ 0.70 10.2 14.42 98.7 — 0.7 0.6 — 6 Mn₃(VO₄)₂ 3.921.4 0.36 100.0 — — — — 7 AlVO₄ 2.66 6.0 2.21 98.0 — 2.0 — — 8 Ni₃(VO₄)₂0.42 1.8 4.28 97.3 2.7 — — — 9 Co₃(VO₄)₂ 2.38 5.1 2.14 100.0 — — — — 10FeVO₄ 1.96 7.9 4.00 100.0 — — — — 11 Zn₃(VO₄)₂ 4.34 1.0 0.23 100.0 — — —— ^(a)Activity based on overall methanol conversion at 300° C.^(b)Turnover frequency based on methanol partial oxidation products(formaldehyde, dimethoxy methane and methyl formate) at 300° C.

TABLE 3 Prep- SELECTIVITY % aration Methanol Example Catalyst conversion% FA DMM DME MF CO CO₂ 5 CrVO₄ 96.0 90.4 — 0.7 — 8.9 — 3 NbVO₅ 100.090.0 — 2.0 — 8.0 — 8 Ni₃(VO₄)₂ 96.2 94.0 — 1.0 — 5.0 — 7 AlVO₄ 90.5 94.5— 2.0 1.0 2.5 — 9 Co₃(VO₄)₂ 81.0 96.6 — — 1.4 2.0 — 10 FeVO₄ 94.2 95.4 —0.8 0.8 3.0 — 2 AgVO₃ 92.9 89.3 — — 0.6 1.5 8.6

It will be understood that while the invention has been described inconjunction with specific embodiments thereof, the foregoing descriptionand examples are intended to illustrate, but not limit the scope of theinvention. Other aspects, advantages and modifications will be apparentto those skilled in the art to which the invention pertains, and theseaspects and modifications are within the scope of the invention.

1. A process for producing formaldehyde from a methanol-containing gasstream which comprises combining said methanol-containing gas streamwith oxygen, at a methanol to oxygen mole ratio of about 0.5 toestablish oxidizing conditions, contacting said combination of methanoland oxygen with a bulk metal vanadate catalyst consisting essentially ofa bulk metal vanadate selected from the group consisting of PbV₂O₆,Pb₂V₂O₇, NaVO₃, Na₃VO₄, Na₂V₆O₁₇, BiVO₄, Bi₄V₂O₁₁, AlVO₄, FeVO₄,Mn₃(VO₄)₂, Mg₂(VO₄)₂, Mg₂V₂O₇, MgV₆O₁₇, MgV₂O₆, CeVO₄, Zn₃(VO₄)₂,Zn₂V₂O₇, CdV₂O₇, VOPO₄, (VO)₂P₂O₇, KVO₃, K₂V₆O₁₇, (NH₄)₂V₆O₁₇, NH₄VO₃,BaV₆O₁₇, TI₃VO₄, TIVO₃, TIV₃O₈, TI₃V₅O₁₄, TI₄V₂O₇, NbVO₅, Nb₂V₂O₉,Nb₉VO₂₅, CrVO₄, Ni₃(VO₄)₂, Co₃V₂, Co₃V₂O₈, AgVO₃, Ag₃VO₄, Cu₃(VO₄)₂,Cu₂V₂O₇, Cu₃V₂O₈, CuVO₃, CuV₂O₆, SbVO₄, VSbO₄, Sn_(1−x)V_(x)O₂,Ti_(1−x)V_(x)O₂, TiVO₄, TiV₂O₆, TiV₄O₁₀ , SrVO₄, Sr₂V₂O₇, CsVO₃, RbVO₃,ZrV₂O₇, V₃Zr₃O, NdVO₄, SmVO₄, EuVO₄, YVO₄, LaVO₄, ReVO₄, LiVO₃, LiV₃O₈,Ca(VO₃)₂, CaV₆O₁₇, Ca₆V₁₀O₂₈, Ca₂V₂O₇, Hg(VO₃)₂, VOSO₄ and mixturesthereof, said bulk metal vanadate having been produced by precipitationfrom a solution of a soluble vanadate species and a soluble metalspecies to form a precipitate followed by calcination of theprecipitate, and continuing said contacting for a time sufficient toconvert at least a portion of the methanol to formaldehyde, whereinconversion of methanol is over 90.5 percent and selectivity of saidconversion to formaldehyde is over 89%.
 2. The process of claim 1wherein the bulk metal oxide catalyst is supported on a refractory metaloxide.
 3. The process of claim 1 wherein said contacting is conducted ata temperature between 200° and 700° C.
 4. The process of claim 3 whereinsaid contacting is conducted at a temperature between 275° and 450° C.5. The process of claim 4 wherein said methanol-containing gas stream iscontacted with said catalyst such that between 10⁻² and 10⁵ cubiccentimeters of methanol contacts a gram of catalyst per minute.
 6. Theprocess of claim 5 wherein between 0.1 and 10⁴ cubic centimeters ofmethanol contact a gram of catalyst per minute.
 7. The process of claim1 wherein the methanol conversion is over 96%.